You get 10 or 30 or 50 points because you have no more bonus left.
10 points = beating iron/bronze
30 points = beating silver/gold
50 points = beating platinum/master
By the way my friend is now in masters rank 2 and i have played with him every single game and have done from average to amazing never gone negative and i've been stuck at rank 1 in bronze the whole time the thing you have to do is play the objective not do what you want to do as i have learned. To add to it another friend in our party is in gold rank 5 and im much better than him.